Great doctor sued mentor (10)
I believe the answer is:
tremendous
'great' is the definition.
(thesaurus)
'doctor sued mentor' is the wordplay.
'doctor' indicates anagramming the letters.
'sued'+'mentor'='suedmentor'
'suedmentor' is an anagram of 'TREMENDOUS'.
(Other definitions for tremendous that I've seen before include "Wonderful" , "Immense" , "Excellent; very great" , "Fantastic" , "Splendid!" .)